Field shields for Schottky barrier devices

ABSTRACT

The present invention relates to an improved Schottky barrier device wherein the leakage current present in the reverse bias mode attributed to the presence of an electric field at the Schottky barrier (18) is significantly reduced by the inclusion of one or more field shields (22), P +  -type diffusions located under the metal anode (16) of the Schottky barrier device at the Schottky barrier (18). The P +  -type field shields, which are disposed in a pattern on the surface of the Schottky barrier, reduce the surface electric field present, thereby significantly reducing the leakage current related thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the inclusion of field shields inSchottky barrier devices, and more particularly, to the inclusion of oneor more field shield diffusions at the metal-semiconductor interface toreduce the surface electric field along the interface, therebydecreasing the reverse bias leakage current of Schottky barrier devices.

2. Description of the Prior Art

Schottky barrier (metal-semiconductor) devices, in particular, diodes,are often used in circuits because they have a low forward voltage dropand a very fast reverse recovery time. These properties make Schottkydiodes very useful in applications such as high-speed switching powersupply rectifiers. However, compared with conventional p-n junctiondiodes, Schottky barrier diodes exhibit poor reverse biascharacteristics manifested in an increased leakage, particularly atvoltages approaching breakdown voltage.

In the past, the reverse characteristics of Schottky barrier diodes wereimproved by increasing the breakdown voltage of the device, utilizingp-type guard rings diffused into the n-type semiconductor material, asdisclosed in U.S. Pat. No. 3,541,403 issued to M. P. Lepselter et al onNov. 17, 1970. As disclosed, the guard ring is located in the substrateunder the insulator-metal interface and functions to reduce the edgebreakdown effects existing at the intersection of this interface and thesemiconductor surface. The same guard ring structure is discussed in anarticle entitled "Silicon Schottky Barrier Diode with Near-Ideal I-VCharacteristics" by M. P. Lepselter et al appearing in Bell SystemTechnical Journal, Vol. 47, No. 2, pp. 195-208.

An improved method for forming guard rings in Schottky barrier diodes isdisclosed in U.S. Pat. No. 4,119,446 issued to S. T. Mastroianni on Oct.10, 1978. Here, the metal-semiconductor structure is formed first andthe metal is then used in conjunction with another mask to form a guardring self-aligned with the periphery of the metal.

Although the use of guard rings will improve the reverse characteristicsof Schottky barrier diodes by reducing the edge breakdown effects,relatively large leakage current in the reverse blocking mode will stillexist, due to the presence of a high surface electric field along theplanar metal-semiconductor interface away from the edge of theinterface. This leakage current generally increases very rapidly as thereverse potential is increased and may be several orders of magnitudelarger than the leakage current of a diffused junction diode when theelectric field approaches the silicon avalanche limit.

In order to reduce the Schottky barrier diode reverse leakage current, aSchottky metal (or metal silicide) which has a high barrier potentialcan be utilized. Although this will improve the reverse characteristics,the high barrier potential results in a higher forward voltage drop and,therefore, greater power dissipation than desired. In an alternativemethod, the electric field is reduced at the Schottky barrier when thedevice is under reverse bias, which results in reducing the leakagecurrent. In particular, the electric field is reduced by increasing theresistivity and depth of the N-type silicon cathode (for the case of ametal-N silicon diode). However, this method of decreasing the leakagecurrent will result in an increased series resistance between the anodeand the cathode and thus will again result in an increased forwardvoltage drop. Further, this method is not very desirable in high-voltageintegrated circuit technology since the N-type cathode material may alsobe used as collectors or drains of bipolar or MOS transistors,respectively, and the increased resistivity will adversely affect thecharacteristics of these devices.

There remains to be solved the problem of eliminating the leakagecurrent present in Schottky barrier devices related to the presence of asurface electric field without unnecessarily increasing the forwardvoltage drop of the device.

SUMMARY OF THE INVENTION

The above-described problem is addressed by the present invention whichrelates to the inclusion of field shields in Schottky barrier devicesand, more particularly, to the inclusion of one or more field shielddiffusions at the metal-semiconductor interface to reduce the surfaceelectric field along the interface, thereby decreasing the reverse biasleakage current of Schottky barrier devices without appreciablyincreasing the forward bias voltage drop.

It is an aspect of the present invention to diffuse a plurality ofclosely-spaced P-type regions, referred to as field shields, into anN-type semiconductor substrate (or to diffuse N-type regions into aP-type substrate). The field shields function to modify and thus reducethe surface electric field at the metal-semiconductor interface(Schottky barrier), thereby reducing the reverse leakage current andonly moderately increasing the series resistance, without increasing theSchottky barrier height or the cathode material resistivity.

Another aspect of the present invention is to utilize a single P-typediffusion region to reduce the surface electric field, where the singlediffusion forms a continuous pattern, for example, a spiral or snakepattern, on the Schottky barrier surface.

Yet another aspect of the present invention is the design of the spacingbetween the field shields as well as the overall pattern of thediffusions such that the reverse leakage current is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 illustrates a cross-sectional view of a prior art Schottkybarrier diode, including a guard ring structure which increases thebreakdown voltage related to edge breakdown effects;

FIG. 2 illustrates a cross-sectional view of a Schottky barrier diodeformed in accordance with the present invention which includes aplurality of field shields to reduce the leakage current associated withthe surface electric field;

FIG. 3 illustrates a top view of an exemplary Schottky barrier diodeformed in accordance with the present invention, where the field shieldsare distributed in a hexagonal array pattern;

FIG. 4 illustrates a top view of an alternative Schottky barrier diodeformed in accordance with the present invention, where the field shieldcomprises a single diffusion disposed in a spiral pattern;

FIGS. 5 and 6 illustrate the calculated electric field contoursassociated with a prior art Schottky barrier diode (FIG. 5) and aSchottky barrier diode formed in accordance with the present invention(FIG. 6);

FIG. 7 illustrates the reverse bias current-voltage I-V characteristicsfor a prior art Schottky barrier diode and a plurality of Schottkybarrier diodes formed in accordance with the present invention; and

FIG. 8 illustrates the forward I-V characteristics for a prior artSchottky barrier diode and a plurality of Schottky barrier diodes formedin accordance with the present invention.

DETAILED DESCRIPTION

In order to aid in the understanding of the present invention, theproperties of a prior art Schottky barrier diode will be brieflyexplained with reference to FIG. 1. As shown, an exemplary prior artSchottky barrier diode comprises an N⁺ -type cathode layer 10 upon whichis deposited an N⁻ -type substrate region 12. An insulating layer 14 issubsequently deposited on N⁻ substrate region 12, where a centralportion of insulating layer 14 is then etched away, exposing N⁻substrate region 12. A metallic layer 16, which forms the anode of theprior art Schottky barrier device, is deposited into the opening createdby the etchant and possibly overlaps a portion of insulating layer 14,as illustrated in FIG. 1. Many different metals and alloys may be usedas layer 16, where nickel silicide is considered to be one alternative.It is to be understood that a Schottky barrier diode may also be formedwith a P-type substrate region and the use of an N-type regionthroughout the present discussion is considered to be exemplary only.

As is well known, a Schottky barrier diode differs from a conventionaldiffused p-n junction diode in that Schottky barrier diodes aremetal-semiconductor junction devices, where Schottky-barrier 18 isillustrated in FIG. 1. As previously discussed, prior art Schottkybarrier diodes attempted to improve the reverse operatingcharacteristics by increasing the reverse breakdown voltage which isrelated to edge breakdown effects. This was accomplished by theinclusion of an annular guard ring, illustrated in FIG. 1 (in across-sectional view) as P-type diffusion 20, where the edges whichproduce the breakdown effect are indicated at points A and B. Asdiscussed in the above-cited prior art references, guard ring 20 forms a"protection" p-n diode in parallel with the Schottky barrier diode, thusincreasing the reverse breakdown voltage of the device. The leakagecurrent related to the electric field along the planar region ofSchottky barrier 18 far from guard ring 20, however, is not reduced oreliminated by the inclusion of guard ring 20 in the Schottky barrierdiode.

A Schottky barrier diode formed in accordance with the presentinvention, capable of significantly reducing the surface electric field,which in turn functions to reduce the reverse bias leakage current, isillustrated in FIG. 2. As can be seen, this device differs from theprior art arrangement of FIG. 1 by the addition of a plurality of fieldshields 22, P⁺ -type diffusions disposed in a predetermined geometricpattern inside guard ring 20, where the individual diffusions areseparated from one another by predetermined distances. Although thecross-sectional view of FIG. 2 shows only three field shield diffusions,in the actual practice of the present invention, a large plurality offield shields, for example, hundreds or even thousands, may be diffusedinto substrate region 12 in the opening created by annular guard ring20. Alternatively, field shield 22 could be formed using a single P⁺-type diffusion in the form of a spiral, or any other type of continuouspattern, over the planar surface of Schottky barrier 18.

In accordance with the present invention, the illustrated distance dseparating the plurality of field shields 22, for an embodimentutilizing a plurality of field shields, must be small enough such thattheir respective depletion regions merge together at reverse potentialswell below the avalanche breakdown of the Schottky barrier diode inorder to significantly reduce the surface electric field. In the case ofa single field shield diffusion, the distance separating adjacentportions of the diffusion must also be small enough to allow thedepletion region of the adjacent portions to merge in a similar fashion.Further, the deeper the plurality field shields 22 are diffused into N⁻-type substrate 12, and the closer they are spaced, the larger thetwo-dimensional field-lowering effect will be.

As stated before, the layout of field shields 22 may comprise one ofmany different geometrical patterns, where the chosen pattern affectsthe reduction of the surface electric field along the planar region ofSchottky barrier 18 located between field shields 22, thus alsoaffecting the resultant decrease in reverse bias leakage current. Oneexemplary pattern of field shields 22 is shown in FIG. 3, whichillustrates a top view of an exemplary Schottky barrier diode with theplurality of field shields 22 disposed in a hexagonal array arrangement.In the hexagonal arrangement, the distance, d separating adjacent fieldshield diffusions will be constant. Alternatively, the layout patternmay be a set of concentric rings, where the separation between adjacentrings is designed to provide the desired reduction in reverse biasleakage while not greatly increasing the forward bias voltage drop. Asingle diffusion, as stated above, may also be utilized to reduce thesurface electric field. For example, a continuous snake or spiraldiffusion pattern over the surface of Schottky barrier 18 can be used toreduce this surface electric field. FIG. 4 illustrates one exemplaryembodiment of the present invention where a spiral diffusion pattern isutilized. Other geometric patterns which may be utilized include arectangular array, or a set of long, parallel stripes, where thesepatterns are illustrative only and many other patterns may be utilizedin accordance with the present invention to provide sufficientimprovement in reducing reverse bias leakage current without seriouslydegrading the forawrd bias voltage drop.

Referring now to FIGS. 5 and 6, the effect on the surface electric fieldrelated to the inclusion of the plurality of field shields 22 isdemonstrated. FIG. 5 illustrates the calculated electric field contoursassociated with a prior art Schottky barrier diode, as discussedhereinabove in association with FIG. 1, and FIG. 6 illustrates theelectric field contours associated with a Schottky barrier diode formedin accordance with the present invention at the same reverse biaspotential as associated with FIG. 5. The illustrated electrical fieldcontours correspond to the electric field present in N⁻ substrate region12 as measured in the X (width) and Z (depth) directions, betweenvertical lines 30 and 32 shown in FIGS. 1 and 2. As can be seen, theinclusion of field shields 22 reduces the maximum magnitude of theelectric field at Schottky barrier junction 18, thereby reducing theleakage current associated therewith.

FIG. 7 contains a semi-log graph illustrating the reverse I-Vcharacteristics of both a prior art Schottky barrier diode and aplurality of Schottky barrier diodes formed in accordance with thepresent invention, illustrated as a function of the spacing, W, betweenthe windows in the diffusion pattern used to create field shields 22. Asstated above, and demonstrated in FIG. 7, the closer together theplurality of field shields 22 are spaced, the lower the reverse biasleakage current becomes.

For example, if the reverse bias voltage, V_(r), applied to theparticular conventional prior art Schottky barrier diode used inassociation with FIG. 7 is, approximately, 140 volts, the reverse biasleakage current, I_(r), is approximately equal to 0.2 mA. By including aplurality of field shields, in accordance with the present invention,which are diffused using a diffusion window spacing W=24 microns, thereverse bias current I_(r) decreases from 0.2 mA to approximately 0.04mA. Using a smaller spacing of W=22 microns current I_(r) to anapproximate value of 9 μA. As shown in FIG. 7, a further spacingreduction to W=20 microns yields a current I_(r) of approximately 2.5 μAand W=18 microns results in I_(r) approximately equal to 1.5 μA.

An advantage of the present invention, as stated above, is that thereverse bias leakage current can be reduced without greatly affectingthe forward-bias characteristics of the device. FIG. 8 contains asemi-log current-voltage (I-V) plot of the forward-bias characteristicsrelated to a conventional prior art Schottky barrier diode and aplurality of Schottky barrier diodes formed in accordance with thepresent invention. Assuming a forward bias current I_(f) ofapproximately 100 mA, the conventional Schottky barrier diode willexhibit a forward voltage, V_(f), of approximately 0.32 volts. Utilizinga Schottky barrier diode formed in accordance with the present inventionwhich uses a diffusion window spacing W=24 microns forward voltage V_(f)increases only 0.025 volts to approximately 0.345 volts. A spacing ofW=22 μm, as seen by reference to FIG. 8, does not greatly change forwardvoltage V_(f). Decreasing the spacing to W=20 μm results in increasingforward voltage V_(f) to 0.365 volts, where a further reduction inspacing to W=18 microns increase V_(f) to approximately 0.385 volts.Thus, as seen by reference to FIG. 8, the use of field shields inaccordance with the present invention does not greatly affect theoverall I-V curve of the forward biased Schottky barrier diode, butmerely increases the forward voltage V_(f) somewhat over the entireforward voltage range.

What is claimed is:
 1. A semiconductor device which comprisesasemiconductor layer of a first conductivity type; an insulating layerdisposed on a surface of said semiconductor layer and having an aperturetherein exposing a portion of said semiconductor layer; a metal layerdisposed in said aperture and possibly onto a portion of the insulatinglayer, forming a Schottky barrier with said semiconductor layertherebelow; and a plurality of semiconductor regions of a secondconductivity type in physical contact with both said semiconductor layerand said metal layer, each semiconductor region of said plurality ofsemiconductor regions being separated from one another.
 2. Asemiconductor device formed in accordance with claim 1 wherein at leastone of the plurality of semiconductor regions of the second conductivitytype forms an annular guard ring surrounding the remaining semiconductorregions of said plurality of semiconductor regions, said annular guardring disposed in physical contact with both the metal layer and theinsulating layer, completely underlying the edge of the interfacebetween said semiconductor layer and said metal layer.
 3. Asemiconductor device formed in accordance with claims 1 or 2 whereintheplurality of semiconductor regions of the second conductivity type arearranged in a geometric pattern with the separation between adjacentregions determined by the desired reverse bias leakage current and thedesired forward bias voltage drop.
 4. A semiconductor device formed inaccordance with claim 3 wherein the geometrical pattern is a regulararray.
 5. A semiconductor device formed in accordance with claim 4wherein the regular array is a rectangular lattice.
 6. A semiconductordevice formed in accordance with claim 4 wherein the regular array is ahexagonal lattice.
 7. A semiconductor device formed in accordance withclaim 4 wherein the regular array is a set of long parallel stripes. 8.A semiconductor device formed in accordance with claim 4 wherein theregular array is a set of concentric rings.
 9. A semiconductor deviceformed in accordance with claims 1 or 2 wherein the plurality ofsemiconductor regions of the second conductivity type comprises aplurality of diffused regions.
 10. A semiconductor device whichcomprisesa semiconductor layer of a first conductivity type; aninsulating layer disposed on a surface of said semiconductor layer andhaving an aperture therein exposing a portion of said semiconductorlayer; a metal layer disposed in said aperture and possibly onto aportion of the insulating layer, forming a Schottky barrier with saidsemiconductor layer therebelow; and a semiconductor region of a secondconductivity type in physical contact with both said semiconductor layerand said metal layer, said semiconductor region disposed to form acontinuous pattern adjoining said Schottky barrier and extending awayfrom the Schottky barrier edge into a central portion of thesemiconductor device.
 11. A semiconductor device formed in accordancewith claim 10 wherein the semiconductor region of the secondconductivity type comprises a diffused region.
 12. A semiconductordevice formed in accordance with claims 1, 2, or 10 wherein thesemiconductor layer is silicon and the insulating layer is silicondioxide.
 13. A semiconductor device formed in accordance with claims 1,2 or 10 wherein the metal layer is nickel silicide.